The Law Posted December 18, 2009 Report Share Posted December 18, 2009 A man and a block rest on a frictionless frozen lake. The man has a mass of 50 kg. He stands to the left of the block. If the man climbs on top of the box, the box will: A) Shift to the right and stop Shift to the left and stop C) slide to the right with some constant velocity D) slide to the left with some constant velocity Answer: The center of mass of an isolated system will not move, so in order to move some mass to the right, some mass must be moved to the left. Or, momentum must be conserved, in order to move some mass to the right some mass must be moved to the left. By climbing on top of the bo, the man moves his mass to the right; the box must shift to the left to compensate. Why is the answer B and not C? It's a frictionless lake, won't it continue moving once it's started unless another force acts? Link to comment Share on other sites More sharing options...
supafield Posted December 18, 2009 Report Share Posted December 18, 2009 The only thing I can think of is that these are two opposing forces canceling each other out. The box only moves as much to the left as the man moves to the right. I think this point in better thought of as momentum. They both have 0 momentum at the start and that momemtum must be conserved. So the momentum created by the man moving right is cancelled by the box moving left resulting in a change of position but no resulting momentum? It's a strange question though, and I would have said it kept going due to the frictionless surface, however I got 8 on my last PS section haha. Alternate explanation... the lake is super long, and upon the seasons changing the ice melts and the man on the block falls through the ice effectively stopping it's movement haha. Link to comment Share on other sites More sharing options...
AG22 Posted December 18, 2009 Report Share Posted December 18, 2009 A man and a block rest on a frictionless frozen lake. The man has a mass of 50 kg. He stands to the left of the block. If the man climbs on top of the box, the box will: A) Shift to the right and stop Shift to the left and stop C) slide to the right with some constant velocity D) slide to the left with some constant velocity Answer: The center of mass of an isolated system will not move, so in order to move some mass to the right, some mass must be moved to the left. Or, momentum must be conserved, in order to move some mass to the right some mass must be moved to the left. By climbing on top of the bo, the man moves his mass to the right; the box must shift to the left to compensate. Why is the answer B and not C? It's a frictionless lake, won't it continue moving once it's started unless another force acts? Good question. It should stop because the momentum before was 0, and we know m 1 = m2 since momentum is always conserved (unless it's an inelastic collison). Just sth to keep in mind That's how I got through physics. Keeping this little factoid or that little factoid in mind. HTH. Link to comment Share on other sites More sharing options...
The Law Posted December 18, 2009 Author Report Share Posted December 18, 2009 Ah, yes, initial momentum was 0. I didn't pay attention. Thanks for the help! Link to comment Share on other sites More sharing options...
The Law Posted December 18, 2009 Author Report Share Posted December 18, 2009 Alternate explanation... the lake is super long, and upon the seasons changing the ice melts and the man on the block falls through the ice effectively stopping it's movement haha. This is why I wish it was an entirely written exam. Link to comment Share on other sites More sharing options...
astrogirl Posted December 19, 2009 Report Share Posted December 19, 2009 Yeah, in reality I'm pretty sure the answer would be c, but it's one of those tricky idealized physics situations where the man has some sort of superpower and can get himself on top of the block without actually moving in the horizontal direction. Link to comment Share on other sites More sharing options...
AG22 Posted December 19, 2009 Report Share Posted December 19, 2009 Ah, yes, initial momentum was 0. I didn't pay attention. Thanks for the help! Yeah, no worries Law. I tend to overthink this stuff too. And find that it always leads to my ultimate peril. But what really helped me, was doing EK 1001. The more you do those problems, the more it cements in your mind, and then when you read a ridonkulously difficile passage, you start to pick out on the underlying tricks, for there are only so many ways they can ask you a question in physics. Practice really does make perfect (esp for folks like me, without any physics knowledge). Link to comment Share on other sites More sharing options...
The Law Posted December 19, 2009 Author Report Share Posted December 19, 2009 Yeah, no worries Law. I tend to overthink this stuff too. And find that it always leads to my ultimate peril. But what really helped me, was doing EK 1001. The more you do those problems, the more it cements in your mind, and then when you read a ridonkulously difficile passage, you start to pick out on the underlying tricks, for there are only so many ways they can ask you a question in physics. Practice really does make perfect (esp for folks like me, without any physics knowledge). Totally agreed. EK1001 and EK physics saved my life last time around. Very good resources indeed. Gotta keep it up!!! Link to comment Share on other sites More sharing options...
supafield Posted December 19, 2009 Report Share Posted December 19, 2009 This is why I wish it was an entirely written exam. Yes, but then you would probably be asked how to treat the mans hypothermia as a follow up.... things could get tricky. The solution to that.... invent a scenario with no follow up. Ex: Newton appears from the sky from a hole in the space-time continuum apparently agitated by the fact that his trip confirms many aspects of Einstein's flashy relativistic physics. In anger, he shouts "A frictionless surface!!"..... "I didn't suffer a grade I concussion from that damn giant falling apple, just so you could go ahead an bastardize my theories by negating friction!!" Newton then initiates lasers from his eyes that instantaneously melt the lake. During the process the block shifts to the left. (answer I may have some holes in this answer though, they could fire back New Question: How do you defeat laser Newton and free mankind from a theoretical prison better known as classical physics? Link to comment Share on other sites More sharing options...
neuronix Posted December 19, 2009 Report Share Posted December 19, 2009 Yes, but then you would probably be asked how to treat the mans hypothermia as a follow up.... things could get tricky. The solution to that.... invent a scenario with no follow up. Ex: Newton appears from the sky from a hole in the space-time continuum apparently agitated by the fact that his trip confirms many aspects of Einstein's flashy relativistic physics. In anger, he shouts "A frictionless surface!!"..... "I didn't suffer a grade I concussion from that damn giant falling apple, just so you could go ahead an bastardize my theories by negating friction!!" Newton then initiates lasers from his eyes that instantaneously melt the lake. During the process the block shifts to the left. (answer I may have some holes in this answer though, they could fire back New Question: How do you defeat laser Newton and free mankind from a theoretical prison better known as classical physics? I believe the answer to that is D: slide left with constant velocity. Link to comment Share on other sites More sharing options...
The Law Posted December 19, 2009 Author Report Share Posted December 19, 2009 LOL. Nerds unite? Link to comment Share on other sites More sharing options...
wizy32 Posted December 22, 2009 Report Share Posted December 22, 2009 the answer must be C the surface is frictionless hence the system(the man and the block will continue to move with constant velocity, once in motion the block will not stop ( a force is needed for a change in velocity F=MA) Link to comment Share on other sites More sharing options...
Marty Posted December 27, 2009 Report Share Posted December 27, 2009 Neat question... When thinking about this problem I imagined what would happen if you had a tripod on ice and varied the "spread" of the legs. Each individual leg would move but the centre of mass of the tripod would remain stationary. In this problem I viewed the man and block as being two separated legs which were then brought together. Since the man moved right to get onto the block, the block must have moved left in order to maintain a stationary centre of mass. Each leg moves but the system as a whole remains stationary. I'm quite rusty on my physics and according to my wife I'm never right. Link to comment Share on other sites More sharing options...
The Law Posted December 28, 2009 Author Report Share Posted December 28, 2009 It's just really important to remember initial momentum must equal final momentum, which means since the masses are not dissapearing... the velocity must start and end with zero. I guess it's a golden rule because it'll make problem solving really easy if you remember it and it's applicable to the q! Link to comment Share on other sites More sharing options...
Octavius Posted December 28, 2009 Report Share Posted December 28, 2009 Examkrackers is awesome eh? The best example was a bomb. Once it blows up, all the momentum is conserved because the directionality of the shrapnel (negative and positive directions). A lot is contained in the number 0 . Link to comment Share on other sites More sharing options...
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